1. Field of the Invention
The present invention relates to a magnetic core member for an antenna that is preferably used in a non-contact IC tag or the like, in which RFID (Radio Frequency Identification) technique is used.
2. Description of the Related Art
As a non-contact IC card and a non-contact IC tag such as an identification tag, in which RFID technique is used, a product obtained by electrically connecting an IC chip for recording information and a resonant condenser with an antenna coil have been known. This non-contact IC tag is activated by transmission of electric wave having a certain frequency from the send/receive antenna of a reader/writer to an antenna coil, and it reads information recorded in an IC chip in response to the read command of the data communication of the electric wave, or it determines whether or not it resonates to the electric wave with a certain frequency, so as to conduct identification or monitoring. In addition, a majority of non-contact IC tags are designed to be able to update the read information, or it is possible to write history information into such tags.
As an antenna module used in such an identification tag, Japanese Patent Laid-Open No. 2000-48152 discloses an antenna module produced by inserting a magnetic core member into an antenna coil, which is wound spirally in a plane, such that the above member can be substantially in parallel with the plane, for example. The magnetic core member in this antenna module comprises a material having a high magnetic permeability, such as an amorphous sheet or an electromagnetic steel sheet. By inserting such a magnetic core member into an antenna coil such that the member can be substantially in parallel with the plane of the antenna coil, the inductance of the antenna coil is increased, so as to improve communication distance.
With regard to this magnetic core member, for the purpose of suppressing generation of eddy current and reducing the loss caused by such generation of eddy current, Japanese Patent Laid-Open No. 2004-52095 proposes that 90 wt % or more of iron-base alloy granular powders contained in a magnetic core member used in an RFID antenna are composed of powders having a particle size of 30 μm or less, and that such powders have a specific resistance of 80×18−8 Ωm or more. This iron-base alloy may comprise 6 to 15 wt % of Si, and may also comprise at least one selected from 1 wt % or less of aluminum, 3 wt % or less of copper, 3 wt % or less of nickel, 5 wt % or less of chrome, and 10 wt % or less of cobalt. The above document describes that a Q value of 30 or greater can be obtained using such iron-base alloy granular powders.
Japanese Patent Laid-Open No. 2005-340759 describes that as a result of intensive studies directed towards providing a magnetic core member used in an antenna module, which is able to improve communication distance without increasing the thickness of the module, the inventors have focused on the loss coefficient in the used frequency (e.g. 13.56 MHz) of a magnetic core member, and they have found that the communication distance can be improved without increasing the thickness of the module, by producing a magnetic core member having a product with a certain value or greater of the reciprocal of the loss coefficient and the real part of a complex magnetic permeability. When the reciprocal of the loss coefficient (tan δ=μ″/μ′) indicated by the real part μ′ and imaginary part μ″ of the complex magnetic permeability in the used frequency of a magnetic core member is represented by Q, if a performance index indicated by μ′×Q is set at 300 or greater, the power loss of the antenna module caused by eddy current loss can be reduced. Thus, without increasing the layer thickness of the magnetic core member, the communication distance can be improved.
Japanese Patent Laid-Open No. 2005-340759 discloses a magnetic core member, in which an Fe—Si based alloy is used. This document discloses that the performance index μ′×Q of a magnetic core member, in which an Fe-10 wt % Si—Cr alloy is used, is approximately 2,000.
It is an object of the present invention to further improve the performance index μ′×Q of the above magnetic core member, in which an Fe—Si—Cr alloy is used.